Hard disk drives (HDDs) are becoming more widely used than ever before, due to the explosive growth in the introduction of portable equipment such as laptop computers, portable media players (PMPs), and handsets. As more and more devices incorporate HDDs, the need has become more pressing to protect them from shocks produced by severe impacts when a product that contains one is dropped accidentally. To increase the ability of HDDs to survive such events, their impact resistance must be enhanced.
There are generally two approaches to establishing the necessary impact resistance, active and passive.
Passive approaches have been in use for a long time; they simply cushion the device with impact-absorbing materials, usually rubber or gels. Gels, which tend to be better able to absorb an impact, are more widely used than rubber. However, gels cannot protect devices from damage caused by falls of more than one meter; this precludes their use in portable entertainment equipment. Devices such as handsets, MP3 players, and PMPs need to be protected for a drop of more than 1.5 meters (the average height of a human ear above the ground). A passive approach to HDD protection is described in Naoki Asakawa, “Tech Analysis: HDD for Mobile Phones Withstand 1.5-Meter Drop,” Nikkei Electronics Asia, January 2005, which is hereby incorporated herein by reference in its entirety.
Among active approaches, there are generally two alternatives for protecting HDDs. One is to increase cache memory capacity so that the HDD is in a read or write mode less often. This approach would also tend to reduce power consumption and heating, but it is costly and fails to deal with the impact that can occur should the HDD be in a read or write mode at the instant the fall begins. The second approach is to employ accelerometers to detect a drop and then generate a signal that causes an HDD head to be recalled to a safe zone. If this can occur before the product hits the floor or other stationary surface, a collision between head and platter will be prevented. This approach was first used commercially in a notebook PC released by IBM in October 2003.
A simple model for free fall of an object is depicted in FIG. 1, where the Z axis of the falling object is assumed to be perpendicular to the surface of the earth. In FIG. 1(a), the object is assumed to be stationary, so that the accelerations along the X and Y axes are both zero, hence the force along the Z axis, governed by Newton's second law, will have the value of 1 g (32.174 feet/second/second at sea level), corresponding to the stationary acceleration force due to gravity.
In FIG. 1(b) the object is allowed to fall. The accelerations along the X and Y axes remain the same, zero g, but now the accelerometer that measures acceleration along the Z axis, being accelerated at the same rate as the object to which it is fastened, will record a value of zero g.
A more general case for a falling object is shown in FIG. 2. Here the edges of the cube form arbitrary angles with regard to the orthogonal coordinate system.
In FIG. 2(a), the object is depicted in a generalized arbitrary orientation; its edges form angles α, with respect to the X axis; β, with respect to the Y axis; and γ, with respect to the Z axis. At zero-g acceleration, the voltage output of each axis sensor is VCC/2. Accordingly, the outputs for the three axes will be:Xoutput=VCC/2±[(sensitivity)(1 g)(sin α)]  (1a)Youtput=VCC/2±[(sensitivity)(1 g)(sin β)]  (1b)Zoutput=VCC/2±[(sensitivity)(1 g)(sin β)(cos γ)]  (1c)
“Sensitivity” refers to the output of the sensor per g. For the ADXL320, when powered by +3 V, the sensitivity will be 174 mV/g. If the direction of the detected linear acceleration corresponds with the positive direction of a coordinate axis—X, Y, or Z—its sign will be positive and its output will add to VCC/2; otherwise it will be negative and will subtract from VCC/2.
When the object is dropped suddenly, the accelerations along all three axes become zero because, regardless of the orientation of the object to the coordinate system, no acceleration will be detected along any axis since, as explained above, the accelerometer is accelerating towards the earth at the same rate as the falling body.
For portable equipment, we must also consider any angular acceleration that may be imparted to the object, as shown in FIG. 3.
In order to simplify the calculation of the angular acceleration, the analysis will be confined to the plane determined by the X and Y axes, thereby simplifying the analysis.
If the angular velocity is ω and the radius of rotation is R, then the angular acceleration (AC) is:AC=ω2R  (2)
Therefore, the components of the angular acceleration along the X and Y axes will be:ACX=ω2R sin θ  (3a)ACY=ω2R cos θ  (3b)
So, in reality, the falling body will generally exhibit both linear acceleration and angular acceleration, a combination of the various cases discussed above.
To compute the time that will elapse when an object falls, starting with a velocity of zero perpendicular to the earth at the instant of the fall, we can use the following equation based upon Newton's second law of motion:
                    t        =                                            2              ⁢              h                        g                                              (        4        )            
where h is the height of the fall and g is the gravitational acceleration, 32.174 feet/second/second. To get a sense of the time available to respond to a fall, we can assume a height of 3 feet. Using equation (4), time=432 ms.
Traditionally, the HDD protection algorithm has been based on free-fall modeling, as explained below, in which the outputs of the sensors contained in the accelerometers can be easily captured by a digital oscilloscope or other data sampling system.
A “test sled” can be assembled using two ADXL320 dual-axis accelerometers. The axes of the accelerometers are aligned with the X, Y, and Z axes, as depicted in FIG. 4, thereby providing values of acceleration along the X, Y, and Z coordinates. (The Y1 output is redundant and is not used.) The outputs of the coordinate axes are sampled by a 12-bit ADC contained in an ADuC832 precision analog microcontroller, which integrates the sampled data and feeds it to an internal 8052-compatible core processor. The sampled data is then transferred to the computer—via an RS-232 interface—for analysis.
FIG. 5 shows the sequence of responses sensed by the two sensors. Values X and Y are supplied by one accelerometer, the values Z and Y1 are supplied by the other accelerometer. Note also that the plot is divided into four consecutive intervals labeled: “static,” “rollover,” “free-fall drop,” and “impact.” The sampling interval, shown along the X axis, is determined by the ADC, which is clocked at 200 Hz for each variable, or one sample of each variable every 5 milliseconds. The Y-axis scale represents the values delivered by the 12-bit ADC in an Analog Devices ADuC832 smart-transducer front end, plotted for all four axes.
The test sled, placed at the edge of the table and caused to roll over, imparting angular acceleration—as depicted in FIG. 4—produces the rollover data shown in FIG. 5. (The Z-axis value, apparently not-equal-to-zero-g output in static mode, is caused by the unbalanced installation of the accelerometer.)
When the sled is pushed off the table, the values are all constant near their respective zero levels during this free-fall-drop interval, in line with the above assertion that during the free fall the outputs of all the accelerometers will be zero-g output. It might also be noted that the zero-g output for the accelerometers along different axes in the same time interval is not quite the same.
The traditional HDD protection algorithm is generally based on the data obtained in the arrangement just described. The system monitors the acceleration along the X, Y, and Z axes of the object. If the root-sum-of-squares value calculated from Equation 5 is equal to or less than the threshold value, a signal is sent to the computer associated with the HDD causing the head to park safely before the portable device collides with the floor.√{square root over (X2+Y2+Z2)}≦Threshold  (5)
The choice of the threshold value is governed by the specific response-time requirement, as well as the sensor's parameters—such as sensitivity, sensitivity change due to temperature, operation voltage, noise density, package alignment error, sensor resonant frequency and the working temperature range of the equipment. Normally, the threshold value can be determined from experiments, like the one described above. For example, a designer might choose a threshold value of 0.4 g.
U.S. Reissue Pat. No. RE 35,269 describes a disk drive protection scheme that uses a three-axis accelerometer. This reissue patent and its parent, U.S. Pat. No. 5,227,929, are hereby incorporated herein by reference in its entirety.
As the name implies, a three-axis accelerometer can detect accelerations in each of three axes, typically referred to as the X, Y, and Z axes. The X, Y, and Z axes are typically normal to one another. A drop or fall condition may be detected using a three-axis accelerometer by measuring the X, Y, and Z axis accelerations and computing the square root of the sum of the squares of the X, Y, and Z axis acceleration measurements. If that value is less than a predetermined threshold (say, less than 0.4 g), a drop or fall condition may be inferred and a disk protection mechanism may be activated.
Three-axis accelerometers are typically expensive and large. Also, the above-mentioned algorithm for detecting a drop or fall may fail if the device is spinning as it drops or falls.